In order to study the effect of freezing on lake eutrophication, Lake Ulansuhai, located in Inner Mongolia, was selected as the study object. The ice and water sample were collected respectively by syringe water sampler and ice drill. The total nitrogen, total phosphorus, COD_Cr, chlorophyll-a, ice thickness, water depth and transparency of each sample were measured. The eutrophic state of Lake Ulansuhai before and after freezing were assessed by the means of fractal theory. The results showed that there was a good negative correlation between water depth and ice thickness in Lake Ulansuhai, and the correlation coefficient was 0.54. The trophic state of water before freezing was eutrophication, and changed to heavy eutrophication after freezing. The results also showed that nutrient elements such as nitrogen and phosphorus were ejected to water during the period of ice growth, that is to say, the process of freezing had a good concentrating effect on pollutants in water. This effect also can make pollutants migrate from water to sediment during ice growth process, which made the quality of under-ice environment deteriorate. While the concentrating effect can provide theoretical basis and data support for the sediment dredging and ice-water separating during ice-on period.